Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium, glass, fabric, metal, or other objects as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (a “latent image”).
After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a visible image. Note that the visible image may not be visible to the naked eye depending on the composition of the toner particles (e.g., clear toner).
After the latent image is developed into a visible image on the photoreceptor, a suitable receiver is brought into juxtaposition with the visible image. A suitable electric field is applied to transfer the toner particles of the visible image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.
The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (“fuse”) the print image to the receiver. Plural print images, e.g., of separations of different colors, are overlaid on one receiver before fusing to form a multi-color print image on the receiver.
Electrophotographic (EP) printers typically transport the receiver past the photoreceptor to form the print image. The direction of travel of the receiver is referred to as the slow-scan, process, or in-track direction. This is typically the vertical (Y) direction of a portrait-oriented receiver. The direction perpendicular to the slow-scan direction is referred to as the fast-scan, cross-process, or cross-track direction, and is typically the horizontal (X) direction of a portrait-oriented receiver. “Scan” does not imply that any components are moving or scanning across the receiver; the terminology is conventional in the art.
Various components used in the electrophotographic process, such as belts and drums, can have mechanical or electrical characteristics that result in periodic objectionable non-uniformities in print images, such as streaks (extending in-track) or bands (extending cross-track). For example, drums can experience runout: they can be elliptical rather than circular in cross-section, or can be mounted slightly off-center, so that the radius of the drum at a particular angle with the horizontal varies over time. Belts can have thicknesses that vary across their widths (cross-track) or along their lengths (in-track). Damped springs for mounting components can experience periodic vibrations, causing the spacing between the mounted components to change over time. These variations can be periodic in nature, that is, each variation cycles through various magnitudes repeatedly in sequence, at a characteristic and generally fixed frequency. The variations can also be non-periodic. For example, two cooperating drums with periodic non-uniformities at frequencies whose ratio is irrational will produce a non-periodic nonuniformity between them.
As a printer operates, its components age at different rates and, eventually, fail at different times. Aging of components can result in degradation of the image quality of prints produced. U.S. Pat. No. 7,400,339 to Sampath et al. describes detecting a banding defect and modifying operation of the printer to compensate. Sampath describes that this scheme extends the operational effectiveness of a printer without requiring downtime for service. Various schemes have been proposed for correcting non-uniformities, including U.S. Pat. No. 7,058,325 to Hamby et al., U.S. Patent Publication No. 2008/0226361 by Tomita et al., and U.S. Pat. No. 7,755,799 to Paul et al., all of which measure printed test patches to evaluate image quality. Similarly to Sampath, U.S. Pat. No. 7,382,507 to Wu describes detecting image quality defects in a print and storing information about the defects at a plurality of times to produce a database of defects. The database contents are analyzed to determine the print engine failure that caused, e.g., a banding defect. However, the scheme of Wu requires reconstructing isolated spectra describing each defect from a simplified description of the defect. This reconstruction can omit significant details of the banding that should be corrected.
Moreover, these schemes use calculations on noisy density data from test patches to detect or compensate for banding artifacts and other non-uniformities. Moreover, multiple components in a printer can have individual non-uniformities of different periods, phases, and amplitudes. These non-uniformities interact with each other, producing significant noise in measured density data and rendering detection more difficult. There is a continuing need, therefore, for an improved method of determining the cause of image artifacts in images produced by an electrophotographic printer.